The present invention relates to subterranean operations, and more particularly, to improved casing strings, and methods of using improved casing strings in subterranean operations.
During the drilling of a well bore in a subterranean formation, a drilling fluid may be circulated through a drill pipe and drill bit into the well bore, and subsequently flow upward through the well bore to the surface. The drilling fluid functions, inter alia, to cool the drill bit, lubricate the rotating drill pipe to prevent it from sticking to the walls of the well bore, prevent blowouts by providing hydrostatic pressure to counteract the sudden entrance into the well bore of high pressure formation fluids, and remove drilled cuttings from the well bore. While drilling fluids generally are not settable, e.g., they do not set into hard impermeable sealing masses when static, drilling fluids may increase in gel strength over time. Typically, after a well bore is drilled to a desired final depth, the drill pipe and drill bit are withdrawn from the well bore and the drilling fluid is left therein so as to, inter alia, provide hydrostatic pressure on permeable formations penetrated by the well bore, thereby preventing the flow of formation fluids into the well bore.
A common subsequent step in completing the well bore usually involves placing a pipe string, e.g., casing, into the well bore. Depending upon factors such as, inter alia, the depth of the well bore and any difficulties in placing the pipe string therein, the drilling fluid may remain relatively static in the well bore for an extended period of time, e.g., up to about 2 weeks. During that time, the drilling fluid may progressively increase in gel strength whereby portions of the drilling fluid in the well bore may become increasingly difficult to displace.
Upon placement of the pipe string in the well bore, primary cementing is typically performed whereby the pipe string disposed in the well bore is cemented by pumping a well fluid through the pipe string and into an annulus between the pipe string and the walls of the well bore, thereby displacing the drilling fluid in the annulus. However, if the drilling fluid has developed sufficient gel strength during its residence within the well bore, an operator may be unable to displace all of the drilling fluid with the well fluid. Accordingly, portions of the drilling fluid in the well bore may be bypassed by the well fluid. This is problematic because the drilling fluid generally is not settable; therefore, formation fluids may enter and flow along the well bore, which is highly undesirable.
Operators have attempted to solve this problem by developing settable spotting fluid compositions, inter alia, to displace drilling fluids from well bores promptly after their use. However, these methods often have been problematic. Conventional settable spotting fluids may include blast furnace slag and other hydraulic components that may begin to set at relatively low temperatures, e.g., temperatures less than about 90° F. Also, certain slag-containing settable spotting fluids may be intolerant to contaminants that may be present in the well bore, causing the settable spotting fluids to set prematurely upon contact with well cement. Oftentimes a set retarder may be added to the settable spotting fluid, which may counteract the tendency to prematurely set, but which may enhance the difficulty in predicting the time when the settable spotting fluid will set.